To help diagnose cancers and to help determine therapeutic strategies, it is useful to detect and locate potentially-tumorous tissues. Locating tumor tissue may involve determining mechanical properties of tissue because mechanical properties of tumor tissue are known to differ from mechanical properties of non-tumorous tissue. For example, the elastic modulus (a measure of force needed to deform a material by a certain amount, and thus a measure of the tissue's stiffness) differs between tumorous tissue and surrounding, non-tumorous tissue. To determine the elastic modulus of tissue, stimuli may be applied to the tissue and elasticity determined, e.g., by measuring induced strain, tissue displacement, or Doppler shifts.
To help determine mechanical properties of tissue, the tissue may be vibrated remotely, using dual ultrasound beams, and mechanical properties of the tissue measured. As an ultrasound beam propagates through tissue, its momentum changes, e.g., due to absorption and scatter of the beam. Momentum changes result in a radiation force acting on the tissue through which the beam propagates. Using beams with different frequencies, f1 and f2, that intersect at a focal region, a wave of varying force is produced that is amplitude modulated at a difference frequency fd=f1−f2, producing tissue motion. The magnitude of the tissue motion depends on the radiation force applied and the mechanical properties of the tissue. It has been suggested that this tissue motion produces an acoustic wave that propagates, possibly through other tissue, and that can be detected by an external hydrophone. Such a detected signal is dependent on at least the mechanical and acoustic properties of the tissue at the focal region and the tissue between the focal region and the hydrophone, the resonance characteristics of the hydrophone housing and its surroundings, and interactions at the hydrophone. Thus, significant noise may be introduced and may affect determination of the mechanical properties of the tissue at the focal region when the method described above is used.